1. Field of the Invention
The present invention generally relates to mobile radio communication, especially relates to a method and system for establishing and adding a radio communication branch in a mobile communications system, and further especially relates to such a method and system for establishing and adding a radio communication branch based on a radio quality parameter between a mobile station and a radio base station.
2. Description of the Related Art
Recently, 3GPP (3rd Generation Partnership Project) has proposed the WCDMA (Wideband Code Division Multiple Access) mobile radio communications system based on Code Division Multiple Access (CDMA) technology, which is standardized in the below described Non-patent Document #1.
The WCDMA system has a diversity hand-over function as a function of radio layer 2. With reference to FIG. 1, the diversity hand-over function is explained below. While performing hand-over, a mobile station 11 can radio-communicate over plural radio links (a link is referred to also as a “branch”) through plural radio base stations (21 and 22). A radio control station 31 has a radio layer 2 protocol termination function unit called diversity hand-over trunk (DHT: Diversity Handover Trunk). The radio layer 2 protocol termination function unit performs downlink data copying distribution and uplink data synthesizing (selection synthesizing) at the time of hand-over. The mobile station 11 performs downlink data synthesizing (maximum ratio synthesizing) and uplink data copying distribution. In this manner, the diversity hand-over function can maximize gain and stabilize the communications at the time of hand-over.
Radio communication branch control for the mobile station when it moves is explained below, with reference to FIG. 1. First, the mobile station 11 changes from a single branch status (left hand) with a radio base station 21 to a diversity hand-over status (center) by “adding” a branch with a radio base station 22 at a request from the mobile station 11. Next, the mobile station 11 moves to a single branch status (right hand) with the radio base station 22 by “deleting” or “cutting” the branch with the radio base station 21 at a request from the mobile station 11. The status changing is controlled by a radio control station 31.
When the mobile station 11 is under the radio environment where it can communicate with both the radio base station 21 and the radio base station 22 as shown at the center of FIG. 1, the mobile station 11 can start radio communications under diversity status and can maximize gain and stabilize the communications even when it does not move.
FIG. 2 shows an application system based on the WCDMA system, in which the present invention can be applied. The WCDMA system shown in FIG. 2 includes mobile stations (User Equipment: UE) 11, 12, 13 with a radio communication function, radio base stations (Node B) 21, 22, 23 with a radio communication function with the mobile stations, radio controlling stations (Radio Network Controller: RNC) 31, 32, 33 connected to the radio base stations, respectively, and controlling radio operations, and an exchange station 60 connected to the radio controlling stations and performing exchange operations. These stations provide mobile communication service between the mobile station 11 and the mobile station 12.
A network including the radio base stations 21, 22, 23 and the radio controlling stations 31, 32, 33 is called a “radio access network” (RAN). A network including the exchange station is called a “core network” (CN). In general the core network 80 has plural hierarchical exchange stations and further comprises a storage device such as a home memory for storing subscribers' information. Only one exchange station 60, however, is illustrated for simplicity of explanation, and the explanations of other devices are omitted.
In the system shown in FIG. 2, the mobile station 13 can transfer user data to the mobile station 11. The transferring route is one from the mobile station 13, via the radio base station 23, the radio controlling station 33, the exchange station 60, the radio controlling station 31 and the radio base station 21, to the mobile station 11. This communication route is referred to as an “external branch”.
FIG. 2 also shows a short communication route via transmission devices, not via the exchange station 60. The radio access networks 71, 72 shown in FIG. 2, are provided with transmission devices 41, 42, respectively. The transmission devices 41, 42 are connected to the radio base stations 21, 22, respectively, and include a radio layer 2 protocol termination function as in the radio controlling station and further include a switching function that follows instructions from the radio controlling station or the exchange station. Two transmission devices can be formed as an integrated single device that can be connected to the radio base stations 21, 22. The transferring route of the user data is one from the mobile station 12, via the radio station 22, the transmission device 42, the transmission device 41 and the radio station 21, to the mobile station 11. This communication route is referred to as an “internal branch”, a “private branch” or a “local branch”.
By utilizing the local branch for communication within the area covered by the radio base stations 21 and 22, user data can be transferred on a short route without traveling through the radio controlling stations 31, 32. Mobile communication service providers can achieve transmission cost reduction by utilizing the local branch communication and therefore can offer special low priced services (referred to as “local branch service”) to special subscribers. The local branch service can be applied to indoor area communication within a company user's buildings.
Such a system including the transmission devices 41, 42 is highly feasible especially when using transmission based on IP (Internet Protocol) reported in Non-patent Document #2. This is because IP is a connectionless protocol where data can be transferred based on only originating and destination addresses without considering a route, and therefore user data passing through the transmission devices can be easily controlled in the IP system.
As above explained, the system may include a radio base station that is not connected to the transmission, such as the radio base station 23 provided in an outside area. In this case, even if the mobile station 12 in local branch communication moves to the outside (toward the mobile station 13) and requires the radio controlling station 31 to add a communication branch with the radio base station 23, the branch with the radio base station 23 cannot be added while continuing the present local branch because the radio base station 23 cannot be connected to the transmission device 41. Accordingly, if there is a branch request while communicating on the local branch, the radio controlling station 31 determines whether the requested base station 23 can be connected to the currently used transmission device 41. If it is determined that the requested base station 23 cannot be connected to the presently used transmission device 41, the radio controlling station has to finish the current local branch communication.
When the mobile station 13 sends a request to start local branch communication under a radio environment where it can communicate with both the radio base station 22 communicable with the transmission device 41 and the radio base station 23 incommunicable with the transmission device 41, it is impossible to perform local branch communication under diversity communication status including radio base station 23 because the radio base station 23 cannot communicate with the transmission device 41. The radio controlling station 31 receives the request from the mobile station 13 and determines whether all the radio base stations communicable with the mobile station 13 can communicate with the transmission device 41. If at least one radio base station incommunicable with the transmission device 41 is found in the determination results, the radio controlling station 31 has to reject the local branch communication request. Accordingly, the radio controlling station 31 has to reject the local branch communication request completely, or allows communication starting in the external communication mode without using the transmission device 41.
[Non-patent Document #1]
3GPP Standard Specification S21.101
[Non-patent Document #2]
3GPP Technical Report TR25.933
[Patent Document #1]
JPL 2004-364054
[Patent Document #2]
JPL 2006-108891